1. Field of the Invention
The invention relates to a DC-DC converter; in particular, to a power converter having faster transient response and capable of enhancing the output limitation of output voltage and an operating method thereof.
2. Description of the Prior Art
In recent years, power supply circuits have been widely used in different electronic products, such as portable electronic products, or computer products. The power supply circuit can provide voltage or current conversion or provide power of fixed voltage or current for the electronic products to operate. In the power supply circuits, a power integrated circuit is one of the necessary active components.
In the conventional multi-phase constant on-time (COT) structure, the pulse-width modulation (PWM) output of each phase channel all uses the same on-time calculation circuit to calculate the on-time. When new on-time is triggered, the distributor of the on-time calculation circuit will distribute the on-time to the specified phase channel in order. However, because each phase channel needs to use the same on-time calculation circuit to calculate the on-time of the PWM signals in order, there is only the PWM signal of certain phase channel at high level at the same time, and the PWM signals of the other phase channels are all at low level; that is to say, the on-time duty cycles of the PWM signals of each phase channel will be not overlapped. Therefore, this conventional COT structure will form serious output limitation to the output voltage of the power converter.
For example, as shown in FIG. 1, if the conventional multi-phase COT structure includes 4 phase channels, and the PWM output signals corresponding to the four phase channels are PWM1˜PWM4 respectively, the maximum duty cycle t1˜t4 of the on-times of the PWM output signals of each phase channel is only 25% of the entire duty cycle. If the input voltage is 5V, the output voltage of each phase channel can be only 1.25V or still lower. It forms serious output limitations to the output voltage. In addition, as to the transient response, although the conventional COT structure has faster response than the fixed frequency system, once the output voltage of the constant on-time structure is increased, the advantage of the conventional constant on-time structure will be disappear even reversed.